1. Field of the Invention
This invention relates to a method for quantitatively determining amounts of calcium in bones of a human body, or the like, and an apparatus for carrying out the method. This invention particularly relates to a method for quantitatively analyzing bone calcium, wherein stimulable phosphor sheets are used and an energy subtraction processing technique is utilized, and an apparatus for carrying out the method.
2. Description of the Prior Art
Quantitatively determining amounts of calcium in bones is necessary for preventing fractures of bones. Specifically, by investigating small changes in the amounts of calcium contained in bones, osteoporosis can be found early, and fractures of the bones can be prevented.
Therefore, various methods for quantitatively determining amounts of calcium in bones have been proposed and used in practice. Such methods are listed below.
i) Microdensitometry (MD method):
With the MD method, a middle finger bone and an aluminum step wedge (a step-like pattern) are simultaneously exposed to X-rays, and an X-ray image is thereby recorded which is composed of a pattern of the middle finger bone and a pattern of the aluminum step wedge. The image density of the X-ray image is then determined by using a densitometer. Thereafter, the X-ray absorption amount of the middle finger bone is calculated with reference to the pattern of the aluminum step wedge and corrected in accordance with the width of the bone. In this manner, the amount of calcium in the bone is determined quantitatively. This method can be carried out with a simple apparatus. However, this method has the drawbacks in that the accuracy, with which the amounts of calcium in bones are determined quantitatively, cannot be kept high, and amounts of calcium in vertebral bones, which very well indicate the sign of an osteoporosis, cannot be determined.
ii) Single photon absorptiometry (SPA method):
With the SPA method, .gamma.-rays having a low energy level are irradiated to a bone, and the .gamma.-rays, which have passed through the bone, are detected by a scintillation detector spaced approximately 15 cm away from the bone. An analog calculation is made from a change in the count of the .gamma.-ray quanta, and the weight of the bone per unit length is thereby calculated. With this method, amounts of calcium in bones can be determined more accurately than the MD method. However, this method has the drawbacks in that amounts of calcium in vertebral bones cannot be determined, a particular management must be done during the use of a radioisotope, and the source of the radiation must be exchanged frequently because of its half-life.
iii) Dual photon absorptiometry (DPA method):
With the DPA method, a nuclide, Gl153, which has two energy peaks of 44 keV and 100 keV, is employed as a radiation source. The amount of calcium in a bone is determined from a difference between the amounts of the two types of radiations having different energy levels, which have passed through a bone. This method is advantageous in that amounts of calcium in lumbar vertebrae and cervixes of thighbones can be determined, and the amount of calcium in bones of the whole body and the amount of fat of the whole body can be determined accurately. However, this method has the drawbacks accompanying the use of a radioisotope. Also, in order for radiation to be scanned, a long time is taken for the inspection to be carried out (for example, a time of more than ten minutes is required when the sample is a lumbar vertebra, and a time of 30 to 40 minutes is required when the sample is the whole body).
iv) Quantitative digited radiography (QDR method or DPX method):
The QDR method comprises nearly the same steps as those of the DPA method, except that, instead of a radioisotope being used, a pulsed X-ray source is combined with a filter in order to yield two types of radiation having different energy levels. With this method, good reproducibility can be obtained, and the inspection time can be kept comparatively short (approximately one third of the time required in the DPA method). This method is most advantageous from the point of view of simplicity and performance. However, the inspection time required is not very short (e.g. a time of as long as approximately six minutes is required when the sample is a lumbar vertebra), and should be kept shorter.
v) Quantitative computer tomography (QCT method):
With the QCT method, an X-ray CT scanner is used, and the amount of calcium in, primarily, the third lumbar vertebra is determined on the basis of the CT number. In order to carry out this method, a large-scaled apparatus must be used.
vi) Dual energy quantitative computer tomography (DQCT method):
The DQCT method is carried out in the same manner as that in the QCT method, except that two types of radiation having different energy levels are utilized and energy subtraction processing is carried out. This method is advantageous in that quantitative determination can be carried out which is free of adverse effects of fat contained in bone tissues. However, in order to carry out this method, a large-scaled apparatus must be used.
As described above, the conventional methods for quantitatively analyzing bone calcium, which are simple, have the problem in that the accuracy of determination cannot be kept high. Also, the conventional methods for quantitatively analyzing bone calcium, wherein the accuracy of determination can be kept high, have the problems in that large-scaled apparatuses must be used to carry out the methods and a long time is required for the determination to be carried out.